Advancing Our Knowledge of Hockey Through Statistical Analysis

Why corsi/possession should not be the goal

Let’s imagine a sport where two factors are equally correlated with winning so that FactorA is 50% correlated with winning and FactorB is 50% correlated with winning. Now for years general managers in this sport only ever knew that FactorA existed and when choosing how to build their team they only ever considered FactorA. Now let’s assume that in this idealist, yet uninformed about FactorB, world every general manager of every team allocated their financial resources perfectly based on their knowledge of Factor A. On top of that, every team is working under the same financial constraints meaning they spend the exact same amount of money.

The result is, in this fictional world, FactorA becomes perfectly evenly distributed across every team. Strangely though, even after accounting for luck, teams have statistically significant differences in winning percentages.

Now, along comes a smart individual who discovers the existence of FactorB and finds out that FactorB correlates 100% with winning percentage (after factoring out luck) and concludes that General Managers were wrong all along and that FactorB is all that matters to winning and FactorA is irrelevant (has to be since it has zero correlation with winning). Upon discovering this he gets hired to become a General Manager of a team and while every other GM was only signing FactorA players he chose to go out and sign solely FactorB players. He made signing FactorB players his goal. Strangely, despite FactorB seemingly showing a 100% correlation with winning, his team didn’t win any more than anyone else.

The reason for this is that FactorA is in fact important. It just doesn’t seem important because everyone knows about FactorA and FactorA is getting evenly spread out across teams. Ignoring FactorA for FactorB is equally wrong as ignoring FactorB for FactorA. Upon learning of the existence of FactorB and its high correlation with winning, the goal of a General Manager is not to optimize his team for FactorB but to recognize that there is undiscovered value in players that have FactorB as a skill while not ignoring other skills that we previously knew existed.

most teams don’t give out contracts because of Corsi. But a team that does will get more wins out of their budget than a team that follows the conventional path and overvalues finishing talent.

My response is, not if it comes at the expense of ignoring finishing talent. Based on Tom Awad’s work, finishing talent is probably at least 50% of out scoring your opposition (note that shooting percentage is a combination of out finishing and shot quality in Awad’s terminology).

So, if teams have been doling out contracts based on, effectively, shooting percentage then it is perfectly reasonable to assume that shooting percentage talent is more evenly distributed across teams than corsi-talent is. Under these circumstances corsi would be highly correlated with winning percentage because that is where the differences lie between teams. This doesn’t mean that corsi is the main factor in out scoring the opponent though and valuing corsi at the expense of shooting percentage will be a detriment to any General Manager.

Furthermore, if General Managers as a whole started paying primarily for corsi we will start to find that corsi talent becomes more evenly distributed across teams and thus shooting percentage would become much more highly correlated with winning (even after adjusting for luck). Furthermore, paying players based on corsi would potentially lead to players altering their style of play to optimize their corsi statistics to the detriment of the ultimate goal, out scoring the opponent.

It is certainly possible in the current hockey universe in which players are paid more by shooting percentage than corsi that they play a style of game to optimize shooting percentage at the expense of winning so it is not unreasonable to see the flip side occur of corsi because a metric by which general managers dole out contracts.

Ultimately, the goal of any General Manager is to optimize his line up for out scoring the opposition, not out shooting percentage-ing them and not out corsi-ing them. Corsi or possession should never be considered the goal just as shooting percentage or any other identifiable skill shouldn’t be. The goal has been, is, and always will be out score the opposition and it’s the General Managers job to find the right balance of all the identifiable skills, not just those that seemingly correlate with winning.

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Welcome to HockeyAnalysis.com, where I strive to get a better understanding of the game of hockey through the use of statistical analysis. I hope you enjoy whatever time you spend here and maybe even learn a little. If you have any questions or comments, feel free to drop me an e-mail at david (at) hockeyanalysis.com.